Oil-charging history constrained by biomarkers of petroleum inclusions in the Dongying Depression, China

Free oils and inclusion oils were extracted from the third member of the Eocene Shahejie Formation (Es3) reservoir sandstones of the wells Chun 51 and Liang 28 in the Dongying Depression, China, and then analyzed by gas chromatography-mass spectrometry (GC-MS). Results show that biomarker distribution characteristics of the free oils and their associated inclusion oils are similar in each Es3 reservoir sandstones of the two wells, but the distributions of Pr/Ph ratio, gammacerane, hopane series and C-27-C-29 steranes in well Chun 51 oils are strikingly different from those of well Liang 28 oils. However, the C(29 )sterane beta beta(beta beta + alpha alpha), Ts/(Ts + Tm) and C-31 hopane 228/(228 + 22R) ratios of inclusion oils are higher than those of related free oils in all the samples, this may indicate that maturity indicators decreased after reaching the maximum and relatively high mature oils were trapped in petroleum inclusions. Comparison with biomarkers of source-rock extracts shows that oils in the Es3 reservoir of well Chun 51 were generated from the upper Es4 source rocks, whereas oils in the Es3 reservoir of well Liang 28 were derived from the lower Es3 source rocks. Moreover, the microthermometric data and burial history indicate one oil-charging episode in the Es3 reservoirs of the two wells, during the late Neogene. It's concluded that different charging histories of these two reservoirs were attributed mainly to the sealing effect of the fault.